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Published online by Cambridge University Press: 30 March 2016
The 21-cm line width is a good indication of the maximum rotational velocity in a galaxy. However, the turbulent motions affect the observed line width and must be corrected. Not taking them into account introduces both accidental and systematic errors.
A first example of this effect is the dependence of the slope a of the Tully-Fisher relation:
on the level at which the line width ΔVobs. is measured. In their 1977 paper, Tully and Fisher find the following values:
A second example is the departure from linearity in the H-band Tully-Fisher relation obtained by Aaronson et al. (1982) (their figure 2): the narrow lines are more strongly affected by these turbulent motions.